Conventionally, a variety of proposals have been advanced on ceramic materials having a function such as a photocatalyst function, an electrical function, a thermal catalyst function or a catalyst-supporting function or environment resistance such as oxidation resistance, alkaline resistance or wear resistance.
For example, with regard to the photocatalyst function of a semiconductor typified by titanium dioxide, it is carried out to form a titania film on the surface of a base material. As a method for forming the titania film on the surface of the base material, there are a sol-gel process and a binder process.
The sol-gel process is a method in which a sol of an organic titanium, which is a precursor for titania, such as titanium alkoxide or titanium chelate is applied to the surface of a heat-resistant base material such as a glass or a ceramic with a spray or the like, the applied sol is dried to form a gel, and the resultant base material with the gel is heated up to 500° C. or more, thereby forming a strong titania film. Titania particles are present on the entire surface of the base material so that the formed titania film can have a high decomposing power and high hardness.
The binder process is a method in which titania particles are fixed on the surface of a base material with a binder. The binder process uses inorganic binders such as silica or organic binders such as silicon.
In the sol-gel process, heat treatment is carried out for converting an organic titanium, which is a precursor for titania, such as titanium alkoxide or titanium chelate into a titania film. Since the crystal form of titania converts from anatase into rutile at a high temperature, the heat treatment is carried out at 500-700° C. at most. For this reason, there is a problem in that the adhesion between the titania film and the base material is insufficient.
In the sol-gel process, in addition, the number of times the aforesaid organic titanium is applied is large and it requires much labor and time. Further, the sol-gel process requires expensive facilities and the cost required is therefore high. Moreover, harmful wastes are generated.
In the binder process, it is necessary to use, as a binder, a material which has high adhesion to a base material and is not affected by the decomposing function of a photocatalyst, so that a problem is that the selection of binder affects the effect.
Further, the binder process has a problem in that the hardness of the titania film formed is low. It is possible to improve the hardness of the titania film formed according to the binder process by increasing the amount of the binder so as to increase the adhesive power. In this case, the amount of titania becomes relatively small when compared with the amount of the binder and therefore the decomposing power descends. In contrast, when the amount of the binder is decreased, the amount of titania exposed on the surface of a base material increases and therefore the decomposing power increases. However, in this case, the adhesive power decreases so that the titania film is apt to be peeled off and the hardness decreases.
On the other hand, concerning a fiber-reinforced ceramic-based composite material or a particle-dispersed type ceramic material, it has been carried out to coat the surface thereof with a ceramic material such as zirconia in order to impart environment resistance such as oxidation resistance, alkaline resistance or wear resistance.
However, for example, when a base material of SiC is coated with zirconia by the sol-gel process, the adhesion between the base material and the zirconia film is not sufficient and, in addition, a crack or a defect easily occurs.
It is an object of the present invention to overcome the above problems and provide a ceramic thin film coating material having a slope constitution which material is excellent in the adhesion to a base material and free from cracks and defects and not only has excellent functions such as a photocatalyst function, an electrical function, a thermal catalyst function, a catalyst-supporting function, etc., or environment resistance such as oxidation resistance, alkaline resistance, wear resistance, etc., but also has excellent dynamic properties, and a process for the production thereof.